1. Field of the Invention
The invention relates to video image capture apparatus including means for compensating for the effects of optical imperfections.
2. Description of the Prior Art
A high definition television system, for example with a resolution of 1920 pixels by 1035 active lines per frame makes very high demands on image capture apparatus such as cameras. High accuracy is required in capture resolution and also in picture geometry and registration. The resolution limits tend primarily to be a function of a sensor arrangement (e.g. a Charged Coupled Device (CCD) sensor or a tube target) of the capture apparatus whereas the accuracy of a picture geometry and registration is primarily determined by the capture apparatus's optical system through which light has to travel before reaching the sensor arrangement.
Significant distortion of the picture geometry can be introduced into a picture output from a camera, for instance, even when high quality lenses are used. Modern television cameras are typically provided with zoom lenses which may work over a relatively large optical range and fields of view. Due to the different optical and mechanical arrangements of the optical system contained in a zoom lens for different zoom settings, it is usually only possible to provide a highly accurate geometric arrangement at one point in the centre of range of operation of the lens. The better the lens, the less will be the distortion generated at other positions. However, it is not practically possible to make a zoom lens which is optically perfect at all points within the optical range. Typically, the design of a lens includes compromises to avoid excessive distortion at any particular point over its range of zoom operation. Similarly, compromises have to be made to avoid excessive distortions over the focusing range of a lens, whether a zoom lens or a lens of fixed focal length.
Other distortion effects can occur due to the placement and alignment of the image sensor or sensors in a camera, for example in a colour camera where a beam splitter is employed. These effects occur with fixed focal length lenses or zoom lenses.
As well as geometric distortions which can be introduced in the optical path through a lens (which may include many optical elements), chromatic distortions may appear at the image sensor. This is because the optical elements in the lens affect different light frequencies differently. Traditionally, a balance has to be met between the geometrical and registration errors which are considered unacceptable, and the expense of the lens. The tighter the specification of the lens (i.e. the lower the level of geometric, registration and chromatic errors which are acceptable), the more expensive it will be. It is to be noted that the cost of a lens in a camera system, for example a zoom lens on a video camera, is a significant portion of the total cost of the camera system, particularly where a very tight specification is required.
FIGS. 1A, 1B, 1C and 1D of the accompanying drawings illustrate four well known examples of geometric distortions which can be introduced into a picture by an optical system.
FIG. 1A illustrates the so called pin-cushion effect.
FIG. 1B illustrates a skew effect.
FIG. 1C illustrates barrel distortion.
FIG. 1D illustrates tilt distortion.
The distortions which occur in a lens system can be characterised to a first approximation by combination of linear distortion (tilt and/or skew) and second order parabolic distortion (barrel or pin-cushion distortion).
Tube cameras are known where electronic means are employed to approximate a correction to a picture distortion by applying inverse signals to the horizontal and vertical scan drives of the tube to cancel the distortion. In other words, rather than attempting to arrange for the horizontal scans to be as near to linear as possible, correction signals are used to modify the output of the horizontal and vertical scan drives to compensate for the distortion encountered. For example, with regards to FIG. 1A, if pin-cushion distortion is caused by the optical system, then the horizontal and vertical scan drives of the tube can be controlled so that the electron beam traces a pattern to compensate for the distortion, so that signals can be output from the tube camera which would reflect the original image without the distortions introduced by the optical system. Such electronic means are widely used in current tube cameras. However, these cancelling signals are normally generated in analogue circuitry and cannot compensate for distortions which are not characterised by combinations of first and second order effects. Moreover, the techniques currently used are only applicable to cameras with image sensors which employ a scanning beam.
The object of the invention is to provide a video image capture apparatus such as a video camera with means for compensating for the effects of imperfections of the optical system of an image capture apparatus which is not limited in application to cameras with image sensors employing scanning beams. For instance, the image sensor or sensors of the video camera might include CCD chips.